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US 4,791,379 AGranted
Electrical filter with adjustable RC product
AssigneeU.S. Philips Corporation
InventorJohn B. Hughes
Life of this patent
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AssigneeU.S. Philips Corporation
InventorJohn B. Hughes
Estimating the term…
A continuous time electrical filter fabricated as an integrated circuit includes capacitors (CO,CN) and resistors (R1,R2). Since capacitors and resistors are difficult to integrate with accurately defined values a trimming circuit is provided which operates switches (S1-SN) to select appropriate ones of the capacitors (CN) to accurately define the cut-off frequency of the filter. The trimming circuit comprises a capacitor (TC2) which is charged through a resistor TR1 during a first period and which is discharged in incremental steps by a capacitor (TC1). The number of incremental steps is counted by a counter (11) and transferred to a register (13). The outputs (S1-SN) of the register control the switches (S1-SN). Instead of adjusting the value of the capacitors in the filter, the values of the resistors may be adjusted. If this is done a convenient procedure is to short out selected portions of the resistors. More than one capacitor or resistor may be adjusted using a single counter and register.
An integrated electrical filter comprising at least one filter section, said filter section comprising at least one capacitor and at least one resistor coupled to an input terminal and an output terminal, switching means for forming a product of the resistance of the resistor and the capacitance of the capacitor, and a trimming circuit for operating the switching means to adjust said product of resistance and capacitance, characterised in that the trimming circuit comprises means for charging a further capacitor from a reference voltage source through a further resistor for a first period, means for removing the charge from the further capacitor in discrete increments of charge during a second period, means for counting the number of charge increments required to remove the charge accumulated on the further capacitor during the first period, and means for operating the switching means to adjust the capacitance of said at least one capacitor or the resistance of said at least one resistor in dependence on the number of charge increments counted.
Dependencies are inferred from the claim text.